Herein, the terms "end-of-life" and "end of the useful life" of a low-pressure arc discharge lamp are defined as that time when the electron-emissive material on one electrode filament has been depleted causing the arc discharge to destroy the filament and/or strike other parts of the electrode mount structure.
Low-pressure arc discharge lamps, such as fluorescent lamps, are well known in the art and typically include a pair of electrodes made of a coil of tungsten wire upon which is deposited a coating of an electron-emissive material consisting of alkaline metal oxides (BaO, CaO, SrO) to lower the work function of the cathode and thus improve lamp efficiency. With electron-emissive material disposed on the electrode filament, the cathode fall voltage is typically about 13 volts. However, at the end of the useful life of the lamp when the electron-emissive material on one of the electrode filaments becomes depleted, the cathode fall voltage rises by 100 volts or more. If the external circuitry fails to limit the open circuit voltage across the lamp, the lamp may continue to operate with the additional power being deposited at the lamp electrode region. By way of example, a lamp which normally operates at 0.1 amp would deposit 0.65 watt at each electrode during normal operation. At end-of-life, the depleted electrode may consume 7.5 watts due to the increase in cathode fall voltage. This extra power can lead to excessive local heating of the lamp and fixture.
Low-pressure arc discharge lamps, especially those designed for operation at high current (1.5 amp) loading, such as very high output (VHO) lamps, sometimes fail by causing the fracture of the glass envelope. It is believed the sequence of events leading to such failures is as follows. At the end of the useful life of the lamp, the electron-emissive material on one of the electrode filaments becomes depleted. When such depletion occurs, the arc discharge strikes other components of the electrode structure and, in particular, the arc strikes the electrical leads supporting the electrode structure. The electrical leads are heated by the arc to the point where the wires soften and bend. Subsequently, the electrical leads and the electrode structure sag and come in contact with the glass envelope. The severe heat generated by the arc and the heated electrode structure causes the glass envelope to fracture.
Various internal structures for low-pressure arc discharge lamps have been proposed which cause the lamp to fail without fracture of the glass envelope. Such structures are suggested in the following references.
U.S. Pat. No. 3,265,917, which issued to Ray on Aug. 9, 1966, discloses a structure comprising a wire or conductive coating electrically connected to the inside portion of the electrode structure and extending to a thin-walled portion of the glass stem press. Upon depletion of the electron-emissive material on the electrode filament, the arc strikes and follows the conductive path reaching the thin-walled portion of the stem press. The heat generated by the arc and the heated conductor softens and melts the thin wall of the stem press to the point where the hermetic seal is lost. The introduction of the external atmosphere into the lamp extinguishes the arc discharge and renders the lamp inoperable.
U.S. Pat. No. 4,105,910, which issued to Evans on Aug. 8, 1978, discloses a structure providing for an auxiliary source of amalgam and for end-of-life extinguishment of the arc. This structure comprises a coating of a suitable amalgamative metal on portions of the stem press and the inside lead-in wire about the point where the lead-in wire emerges from the stem press.
U.S. Pat. No. 4,495,440, which issued to Schlitt et al, discloses an arc-extinguishing ampul mounted on each electrode structure. The ampul comprises a thin-walled glass body enclosing an arc-extinguishing gas, at least one electrically conductive support wire, and a heat-conductive coating covering the outer surface of the ampul and portions of the support wire. Upon depletion of the electron-emissive coating on one electrode filament at the end of the useful life of the lamp, the arc discharge is attracted to the ampul by the support wire. The heat of the arc softens and melts the ampul to the point where the arc-extinguishing gas within the ampul escapes and renders the lamp inoperable without loss of the lamp's hermetic seal.
Although the above-described end-of-life structures have been employed with varying degrees of success, it has been found that certain disadvantages do exist and such structures do leave something to be desired. More specifically, the above-described wire, conductive coating or amalgamative metal require a stem press mount configuration to implement. On the other hand, the arc-extinguishing ampul is rather expensive from a lamp-manufacturing standpoint because it adds additional manufacturing steps to the lamp process. Therefore, it would be very desirable and advantageous to have an end-of-life structure that does not require a stem press mount and which is more economical to utilize.